Filamentous bacteria - moving from identification to discussing causation

Gram stain activated sludge (400x magnification)

Filamentous form microorganisms form an important part of biofilm/floc ecology, yet are usually universally derided for their tendency to not settle/compact when in overabundance. The filamentous organism has more surface area per unit volume relative to floc forming organisms. This gives the filamentous bacteria its unique strengths yet also is the root of the bulking problem.

First - let's mention the good things about filamentous bacteria:

• Usually quite effective at removing organics in wastewater
• A number of filaments are effective at removing sulfides
• Form the "backbone" of good floc providing needed strength
• Good growth with low D.O. and nutrients

Now, the bad:

• ​Some filaments cause foaming (Nocardia, M. parvicella)
• Can create bulking where sludge does not compact upon settling

Now may facilities send their bulking MLSS samples for filamentous identification. While this should be done periodically, most causes of filamentous bulking can be seen by looking at operational data. The microscopic ID process just confirms what is already known. In my opinion, filaments can be grouped by the major factors favoring their growth. Some filaments are found in multiple groups and are among the most common that I see in industrial wastewater. Here are my groups (again many filaments are in more than one group):

1. Low D.O. 
2. Low nutrients(N,P) 
3. Short-chain organic acid
4. Sulfur oxidizing
5. Grease utilizing (foaming) - usually long MCRT organsms